An example of an air conditioning system that is divided into an outdoor unit and an indoor unit is shown in FIG. 4. The air conditioning system 101 has one air-cooled outdoor unit 102 and a plurality of (more specifically, three) indoor units 103, 104, 105 and is used to air-condition an office or the like. The outdoor unit 102 is equipped with a compressor 111 and an outdoor heat exchanger 112 and is installed outdoors. The indoor units 103, 104, 105 are each equipped with an expansion valve 113, 114, 115 and an indoor heat exchanger 123, 124, 125 and installed in an indoor room 133, 134, 135. The outdoor heat exchanger 112 and the expansion valves 113, 114, 115 are connected together by a liquid refrigerant pipe 116. The indoor heat exchangers 123, 124, 125 and the compressor 111 are connected together by a gaseous refrigerant pipe 117.
In this air conditioning system 101, as shown in FIGS. 4 and 5, the gaseous refrigerant is compressed by the compressor 111 from the state at point A0 to a prescribed pressure Pd0 (see point B0 in FIGS. 4 and 5) before being delivered to the outdoor heat exchanger 112. In the outdoor heat exchanger 112, the gaseous refrigerant exchanges heat with the outside air and condenses, changing to a liquid refrigerant state (see point C0 in FIGS. 4 and 5). This condensed liquid refrigerant is delivered from the outdoor heat exchanger 112 to the expansion valves 113, 114, 115 of the indoor units 103, 104, 105 through the liquid refrigerant pipe 116 and the pressure of the liquid refrigerant is reduced to Ps0 (see point D0 in FIGS. 4 and 5) by the expansion valves 113, 114, 115. In the indoor heat exchangers 123, 124, 125, the pressure-reduced refrigerant exchanges heat with the air inside each respective room and evaporates, changing to a gaseous refrigerant state (see point A0 in FIGS. 4 and 5). The evaporation temperature of the refrigerant at the indoor heat exchangers 123, 124, 125 is the temperature T0 corresponding to the pressure Ps0. The gaseous refrigerant is drawn into the compressor 111 through the gaseous refrigerant pipe 117. In this way, the air inside the rooms is cooled.
Due to the increased use of computers in recent years, the floor space of offices and the like is often partitioned to provide server rooms for the computers. In this kind of server room, it is necessary to run the indoor unit in cooling mode constantly regardless of the season in order to process the heat discharged by the server equipment.
However, when the outside air temperature is low, such as in the winter, the refrigerant evaporated in the indoor heat exchangers 123, 124, 125 of the conventional air conditioning system 101 partially changes to a liquid (see point E0 in FIGS. 4 and 5) by the time it reaches the compressor 111 through the gaseous refrigerant pipe 117 after leaving the outlets of the indoor heat exchangers 123, 124, 125 (see point A0 in FIGS. 4 and 5). When this partially liquefied refrigerant is drawn into the compressor 111, such problems as damage to the compressor 111 and insufficient intake of gaseous refrigerant occur.
Therefore, conventionally, the openings of the expansion valves 113, 114, 115 are adjusted such that the refrigerant pressure in the indoor heat exchangers 123, 124, 125 is lowered (see point D1 and pressure Ps1 in FIG. 5) and the evaporation temperature of the refrigerant in the indoor heat exchangers 123, 124, 125 is brought to a temperature T1 that is lower than the outside air temperature, thus preventing the gaseous refrigerant from liquefying inside the gaseous refrigerant pipe 117 (see point A1 in FIG. 5).
If the evaporation temperature of the refrigerant is lowered too much, however, the refrigeration cycle of the air conditioning system 101 will be along the lines joining points A1, B1, C1, and D1 in FIG. 5 and the indoor heat exchangers 123, 124, 125 will freeze. As a result, it will not be possible to continue running the indoor units 103, 104, 105. When such a situation occurs, the indoor units 103, 104, 105 are generally run in fan-only mode to increase the temperature of the frozen indoor heat exchangers 123, 124, 125 and return them to an unfrozen state. In a room, such as server room (assume, for example, that room 133 in FIG. 4 is a server room), where the amount of discharged heat is large, the temperature inside the room will rise rapidly when the cooling operation is stopped and the operation of the server equipment could possibly be impeded.